The use of chlorine as a sanitizer or disinfectant for various water supplies and various types of equipment, like food processing equipment and medical equipment, such as a hemodialysis unit, is common. Because the amount of available chlorine in an aqueous solution relates directly to the disinfecting or sanitizing activity thereof, a test which rapidly and accurately measures available chlorine is important.
The available chlorine family is comprised of compounds which, when in aqueous solution, yield solutions of hypochlorous acid. The available chlorine family is further divided into compounds containing free available chlorine and compounds containing combined available chlorine. The sum of free available chlorine and combined available chlorine is termed total available chlorine.
Free available chlorine encompasses chlorine-containing compounds in aqueous solution such as hypochlorous acid, hypochlorite ion, and, in strong acid solutions, free chlorine. The use of free available chlorine as a disinfectant for water supplies and equipment is widespread because of its low cost, convenience, and effectiveness as an antiseptic agent in relatively low concentrations. For example, free available chlorine is used as a disinfectant in a majority of hemodialysis centers.
Combined available chlorine, also termed bound available chlorine, mainly encompasses organic chloramines, which release only a small amount of free available chlorine in aqueous solution. Chloramines are formed from chlorine reacting with amine compounds in water. The amine compounds can be an impurity in the water or arise from ammonia added to water with chlorine during water disinfection. Ammonia and chlorine are added to the water to form chloramines which stabilize chlorine from decomposition and/or evaporation, and also increases the bacteriocidal potency of chlorine. Depending on the ratio of chlorine-to-ammonia and the acidity of the water, chloramines formed from chlorine and ammonia are a mixture of monochloramine, dichloramine, and trichloramine at various ratios. Although monochloramine is the main chloramine of concern due to its toxicity, removal of all chlorine is essential for safe and effective operation of a dialysis water purification system.
Conventionally, combined available chlorine has not been considered an effective disinfectant or sanitizer. Accordingly, prior chlorine assays have focused on assays for free available chlorine, i.e., the active disinfectant. For example, assays disclosed in Rupe et al. U.S. Pat. No. 4,092,115 and Ramana et al. U.S. Pat. No. 5,491,094, consider combined available chlorine as an interferant in the assay for free available chlorine, and the assays have been designed only to measure free available chlorine. However, in some applications, it is important to assay for total available chlorine.
For example, chlorine is used in hemodialysis centers to sanitize hemodialysis units because chlorine is an effective and economical sanitizing agent. It is important to clean and disinfect a hemodialysis unit between each dialysis session to prevent pathogen contamination from patient to patient. However, chlorine also is a very toxic compound that can cause hemolysis even when only a trace amount of chlorine diffuses from the hemodialysis unit into the blood of an individual. Therefore, if an assay for residual chlorine in a hemodialysis unit detects only free available chlorine, a potentially toxic amount of combined available chlorine, which slowly generates free available chlorine, can be present to adversely affect an individual subsequently connected to the hemodialysis unit. Trace amounts of free available chlorine also can adversely affect filtration membranes of the hemodialysis unit.
Combined available chlorine is considered highly toxic because of its electronic neutrality and ability to penetrate cell membranes. In a municipal source water, combined available chlorine always exists in various proportions relative to total available chlorine. Combined available chlorine is formed in a reaction of free available chlorine either with amine compounds, which are present as contaminants in the source water, or with ammonia, which is added to the water with free chlorine to stabilize the chlorine and to increase the bacteriocidal potency of the chlorine disinfectant.
In a dialysis unit, all chlorine species are removed before the water can be used in hemodialysis. Chlorine removal is usually performed by passing the water through a water purification tank containing activated carbon, and then through a reverse osmosis column. The presence of combined available chlorine in the water affects the efficacy of the carbon tank in removing all chlorine species. Knowledge of the concentration and the relative amount of combined available chlorine to total chlorine is important in designing the water purification system, as well as devising a method of monitoring chlorine in the purified water.
Occasionally, a trace amount of chlorine leaks through the tank. If the chlorine leaking through the tank is all, or substantially, combined chlorine, this suggests exhaustion of carbon tank capacity. However, if the chlorine leaking through the tank contains a high proportion of free chlorine, this indicates the presence of a mechanical defect, such as channelling through the activated carbon inside the tank. Determination of both the free and combined available chlorine is important in managing the water purification for dialysis.
Therefore, when a sanitizing solution is used in medical or food processing equipment, two critical chlorine levels must be monitored. First, the free available chlorine concentration must be sufficiently high to perform a sanitizing or disinfecting function, i.e., at least about 1000 ppm (parts per million) free available chlorine is needed to effectively sanitize equipment. Typically, a chlorine concentration sufficient for equipment sanitization is about a 1 to 10 volume dilution of a 5.25% (by weight) sodium hypochlorite with water, to provide a solution containing about 0.5% to about 0.6% (by weight) sodium hypochlorite, i.e., about 5000 to about 6000 ppm chlorine. During the sanitizing process, the sanitizing solution is assayed periodically to ensure that sufficient free available chlorine is present to sanitize the equipment.
After the sanitizing function is completed, and before use, the equipment is rinsed with water to flush residual chlorine from the equipment. The rinse water also is assayed for available chlorine to ensure that the level of residual available chlorine is below the maximum allowable level, e.g., 0.5 ppm as recommended by the Association of Advancement of Medical Instrumentation (AAMI). In practice, the residual available chlorine concentration is essentially zero, or at least below the lowest detectable levels of about 0.1 to about 0.2 ppm, i.e., equivalent to a 1 to 100,000 water dilution of 5.25% (by weight) sodium hypochlorite.
Presently, only two types of commercial assay systems are suitable for assaying hemodialysis units for available chlorine. One assay utilizes tablets or dry powder, and the other utilizes dry chemistry dip strips. Each assay has advantages and disadvantages, and neither assay satisfies the different testing requirements needed for a hemodialysis unit.
The tablet method has good sensitivity (e.g., 0.1 ppm) and is less expensive per assay. However, the tablet method is more cumbersome to perform and requires more technician time. The dry chemistry test strips usually are not as sensitive as the tablet method and can cost more per test. Nevertheless, the strip test is very easy and convenient, particularly when operating a mobile hemodialysis unit. In most hemodialysis centers, the test strip is used as a screening test for residual chlorine, whereas the tablet method is used for more critical water testing. Because of the differences in test requirements, most hemodialysis centers are forced to stock both the tablet and dry chemistry test systems.
To date, no known single assay is available to assay both the high and the low available chlorine concentration range because the 10,000 fold difference in chlorine concentration between a working sanitizing solution and a residual chlorine concentration makes detection and differentiation of concentration levels difficult. The present invention is directed to providing a single assay for total available chlorine that is capable of measuring total available chlorine concentration over the range of 0 to greater than 5000 ppm, and especially 0 to greater than 1000 ppm.
The present invention, therefore, is directed to an assay method and device that can be used to test: (a) a working sanitizing solution, containing 500 ppm or more of free available chlorine, without diluting the solution, and (b) residual chlorine, both free and bound available chlorine, present at 0.5 ppm or less in rinse water. The present invention also is directed to a method of simultaneously determining the relative amounts of free and bound chlorine in a test sample containing 0.5 ppm or less residual chlorine. Such a determination provides important information with respect to the effectiveness of a chlorine solution and whether potentially harmful amounts of residual bound chlorine are present in rinse water.
Accordingly, a test strip can be used either to test for residual chlorine in the rinse water after cleaning the hemodialysis unit, or to test for the available chlorine content of a working solution. As illustrated hereafter, the present test strips have a good sensitivity and a wide detection range with a continuous color response from 0.5 to over 1000 ppm total available chlorine.
The present method of assaying for total available chlorine in an aqueous test sample yields trustworthy and reproducible results by utilizing an indicator reagent composition that undergoes a color transition in response to total available chlorine concentration, and not as a result of a competing chemical or physical interaction, such as a preferential interaction with another test sample component. For example, the present indicator reagent composition has sufficient sensitivity to detect as little as 0.1 ppm total available chlorine. Additionally, the method and composition utilized in the total available chlorine assay does not adversely affect or interfere with any other test reagent pads that are present on a multiple test pad strip. As set forth hereafter, methods are available such that the composition can be used to determine the relative amounts of free and bound available chlorine in a test sample.
In accordance with the present invention, an indicator reagent composition can be incorporated into a carrier matrix to provide sufficient sensitivity and color differentiation to assay for total available chlorine concentration over the range of 0 ppm to greater than 5000 ppm, and typically greater than 1000 ppm. In addition, although dry phase test strips have been used to assay for chlorine concentration, or to assay for the relative amounts of free and bound available chlorine in a test sample, no dry phase test strip has been used to assay for total available chlorine over such a wide concentration range, examples of prior disclosures relating to assaying for chlorine include Storm U.S. Pat. No. 3,718,605; Reiss U.S. Pat. No. 4,938,926; Ross, Jr. et al. U.S. Pat. No. 4,049,382; Frant U.S. Pat. No. 5,300,442, Harp U.S. Pat. No. 5,362,650; O'Brien et al. U.S. Pat. No. 4,904,605; and J. D. Johnson et al., Analytical Chemistry, 40(13), pages 1744-1750 (1969).